Y axis stalling

[richie00boy]

Been running the machine for a while tonight trying to get it to do the stalling. Typical - when I want it to happen it won't! These are my settings. Sure if it was a setting it would be doing all the time?

[John S]

Change your step pulse to 5 or 6 and try that. ignore the direction pulse.

[richie00boy]

Not used the machine for some time until the other week when I did a couple of PCBs. Noticed that it was losing position a bit when drilling the holes as they were not quite central on the pads - I put up a thread about this.

After trying to reface my spoilboard it was stalling like mad on the Y axis again and I think actually this is what caused the PCB holes to be misaligned - the machine is just moving so slow that I can't hear or notice it then.

I turned off the controller and tried turning the stepper by hand using the knob on it, it was pretty stiff. So I decided to take it apart a bit and try Jazz's suggestions.

Well it moves freely on the rails when I unscrew the leadscrew mount. With the leadscrew mount loose I did rapid Y movement (shift and up/down arrow on Mach3) and it runs nice until the mount hits the lip of the gantry and then starts dragging the gantry along. I can see the mount bobbing up and down slightly sometimes. So I'm wondering if the leadscrew is a bit bent? I guess if it is this could be the cause of the problem?

If I do need a new leadscrew where can I get one and is there an upgraded type worth fitting?

[JAZZCNC]

If screw is bent enough to cause stalling then you should see it easy enough.? Bent screw will cause a vibration first then if really bad will cause stalling.
It it is indeed a Lead screw and not ballscrew(Hard to tell from pic) I would look at the lead nut it's self to see if it's tight or badly worn and it's binding under load.! What material is the lead nut made from.? looking at the colour of the lead screw I suspect Delrin.? Can you post a picture of it.

If replacing the screw/nut then really you want a ballscrew. They are far superior to lead screws and if bought from China then will probably cost only slightly more than a decent lead screw bought in this country. They will also machine the ends to your spec if the ends are custom machined, which I suspect they will be on this machine.!

[richie00boy]

I'll have to strip the machine more to get at the leadnut - there is grease on it so I can't see what it's made of. I woudn't have thought it was excessively worn as it's only run about 10 hours.

It does vibrate and stall.

Is the current leadscrew threaded rod that I can buy easily and get the machinist at work to do the ends for me? I don't want to spend more than 50 quid really, but the primary use of the machine is PCBs so I need to get low backlash.

[JAZZCNC]

I'll have to strip the machine more to get at the leadnut - there is grease on it so I can't see what it's made of. I woudn't have thought it was excessively worn as it's only run about 10 hours.

It does vibrate and stall.

Suggest you take it off completely and check screw against flat surface to see how much it's bent. It may be possible to straighten if not bad.

You also need to know why it's bent.? Has it bent because the alignment is off and one end is higher or offset to the other. If so what happens is the leadnut which is fastened to the gantry bends the screw when it approaches the end which is higher or lower than other. The screw fixed ends and leadNut need to be perfectly aligned on the same plane. This is a common cause of bent screws.!!

Is the current leadscrew threaded rod that I can buy easily and get the machinist at work to do the ends for me? I don't want to spend more than 50 quid really, but the primary use of the machine is PCBs so I need to get low backlash.

Like I say hard to see from picture.!! . . . BUT it won't be thread rod like you buy at local hardware store.
It could be what's called Acme Lead screw which is often used on cheaper machines, if this one of the early Chinese machines then chances are it is.? If so they used plastic lead nuts which wore out very quickly and could have done so even with 10hrs use that's how crap they where.!!!!

The later slightly better Chinese machine are coming with 5mm pitch ballscrews, Looking closer at the pics I'm tipping it's lead screw not ballscrew.

You will buy a ballscrew with nut from china complete with end machining for around £50 delivered.
TR14 lead screw will cost about £15-20, lead nut will be £10-15 then you'll need to machine the ends. You could be up and running for £20-£30 but you'll still have crappy screw with some backlash in the nut unless you buy better anti-backlash nut which will cost much more.!!. . . So you can probably tell were I'm going.? Not worth IMO so just Bite the bullet and buy a ballnut you'll get much smoother machine with better accuracy and probably more speed.

[richie00boy]

Thanks for the help. How can I tell if the alignment is off to bend the leadscrew? And is that fixable?

I do remember noticing that the leadscrew was not totally concentric when I first got it. I assumed it was just due to the leadscrew being made from really long thread which got slightly bent at some point in the Chinese factory.

[JAZZCNC]

Thanks for the help. How can I tell if the alignment is off to bend the leadscrew? And is that fixable?

With careful measurements and visual checks.!! Aligning ballscrew and nut is tedious job but very important to both prevent bending and getting the best performance.
If alignment is off enough to bend the screw it should easily be visible by manually running the gantry along it's length carefully watching the screw to see if it's bending. Also if you turn the screw by hand with motor disconnected and you feel it stiffen has it nears ends this is good indication some things off.!!

Can't really comment on what would be needed to fix without seeing how ends are fixed and how much off it is.?

[FatFreddie]

Thanks for the help. How can I tell if the alignment is off to bend the leadscrew? And is that fixable?

If you take one of the bearings off and turn the screw by hand, as the nut approaches the other bearing it should be fairly obvious if it's misaligned, particularly if you can get the nut quite close to the bearing.

[richie00boy]

OK, before I take things apart a bit more and get lost, here is what I have found.

1. The motor knob becomes stiff when the leadnut under the gantry can be at almost any point along the length of the leadscrew, i.e. not just at the ends.
2. I have unscrewed the leadnut from the gantry and can see the leadnut housing bobbing about relative to the gantry again at almost any point along the length of the leadscrew.

I'm thinking that there could be two ways for bending the leadscrew. One if the leadnut was not mounted centrally, and the other if the chassis is out of square or the leadscrew not exactly in the middle of each end piece. How can I check for all of these? Because there is also the possibility that a bent leadscrew has been fitted at the factory - which I'm hoping is the case and my machine is fundamentally sound. Also I would not be that bothered if the leadnut was not mounted centrally because if I convert to ballscrews I would be fitting a new mount.